As methods for connecting pipes, made of stainless steel, copper, or aluminum, for low-pressure piping in line with each other comprising an exhaust pipe of an automobile, a piping used in an exhaust gas recirculation system to return part of exhaust gas flowing in the exhaust pipe to halfway the air intake system, or a piping for cooling water recirculating between the engine and radiator, as well as a water mains piping, a sanitary piping, and a piping for recirculating air conditioner coolant or refrigerant, there have been known a butt welding method in which end faces of the pipes to be joined are placed opposite one another and welded, a bell and spigot welding method in which pipe ends to be joined are inserted into are pipe joint such as a socket and the pipes and the pipe joint are welded, a screw type in which pipe ends to be joined are threaded and the pipes are screwed each other or through a threaded pipe joint such as a socket or nipple, and a flanged type in which flanges are integrally formed at pipe ends to be joined by welding, and the pipe flanges are connected through retaining pieces such as bolts and nuts.
Furthermore, there have also been known improvement of these basic pipe joining methods, for example, as disclosed in Japanese Patent Publication Laid-open 62-199234/1987 and Japanese Patent Publication Laid-open 1-127124/1989, in which, for some reason, pipe ends or sleeves to be joined are complexly formed and connected.
The above-described pipe joining methods have problems in long-term reliability due to the use of welding, screws, or flanges and of low efficiency of the joining work, in addition to the problems shown below.
When a method for joining pipes by welding is adopted, since the surface of pipe joints or pipes is generally stained with lubricating oil, the working environment tends to be deteriorated due to gases or fumes generated by heating during welding, and it is necessary to select welding conditions in consideration of thermal effects on the pipes. Especially, when pipes with relatively thin walls are joined, thermal effects of welding on the pipes must be sufficiently considered, and the welding work itself tends to be very tedious and poor in productivity. Furthermore, portions at the beginning and ending of welding tend to have defects such as pinholes, requiring thorough quality control work and resulting in an increase in production cost.
When joining pipes using threads, it is necessary to use pipes having wall thicknesses in view of deterioration in strength of the threaded portions. Furthermore, since these connections are less air-tight, it is necessary to use a sealing material to improve the air-tightness, which leads to an increase in number of parts or man-hour.
When joining pipes using pipe flanges, it requires a tedious work because pipe flanges are formed at the pipe ends to be joined by welding or the like. Furthermore, it is necessary to use a sealing material between the joining surfaces of the pipe ends or pipe flanges in order to obtain air-tightness, and retaining parts such as bolts and nuts and the like to retaining the pipe flanges each other, which leads to an increase in number of parts.
In the above-described inventions described in Japanese Patent Publication Laid-open 62-199234/1987 and Japanese Patent Publication Laid-open 1-127124/1989, since it is necessary to use complex-shaped sleeves or pipe joint structures, and the joining portions are deformed towards the inside in the radial direction, these inventions have problems in airtightness when applied to thin-walled pipes.